Telephone transmission



TELEPHONE TRASMISSION Filed Sept. 28. 1921 gy@ S/ k Patented May 8,1928.

UNITED STATES FREDERICK WHITTLESEY MUKOWN, OF NEW YORK,

, ELECTRIC COMPANY, INCORPORATED,

NEW YORK. i

N. Y., ASSIGNOR T WESTERN' OF NEW YORK, N. Y., A CORPORATION 0ITELEPHONE TRANSMISSION.

application med september as, 1921. serial No. 503,755.

This invention relates to telephone transmission.

Experiments have shown that the voice frequency range can be dividedinto parts 'l each of which will give a commercial degree ofarticulation. One object of the invention is to utilize the principleinvolved in this discovery for satisfactorily repeating voice currentsin oppositedirections in a v line without regard to conditionsofimpedance unbalance in the line. Another object of the invention is toutilize this principleto enable a plurality of pairsof subscribers totalk over the same pair of lines l5 simultaneously.

'Ihe first of these objects is accomplished by transmitting only a partof the voice frequency range in one direction and transmitting theremainder, or a part of the remainder, in the opposite direction, eachof theseV arts being sufficient to represent intelligi le speech. `Thismethod makes it possiblej to employ 'frequency separation of theoppositely directed transmissions at the repeaters, so that the gain ofthe repeaters is limited not by considerations of line imedance orimpedance unbalance, but only y the efficiency of the filtersor othermeans used to effect the frequency separation, and

furthermore, the necessity for the balancing networks ordinarilyeemployed in `other sys tems is avoided.

To accomplish the second object' stated above one duplex conversation islimited to an intelligible part only of the voice frequency range,transmission in both directions over a line using this part of therange, and the remainder of the range, or a part of the remainder, isused to transmitscription and the appended claims taken in,

connection with the accompanying drawings wherein Fig. 1 is a diagram ofa repeating system employing entirely distinct repeatover the same linea second duplex conversa.-`

ing channels for oppositely directed trans- `missions, `with amplifyingelements individual to the two channels; Fig. 2 is a diagram of amodified repeating system using an amplifying element arranged to serveboth channels; Fig. 3 is a dia am sim1lar to that of Fig. 2, but withlters connected. inv parallel rather than in series as 1n Fig. 2; andFig. 4' is a diagram of a multiplex telephone syste In Fig. 1 a line Wand a line E are connected by] a repeater station S comprising filters Fand FW and amplifiers or repeaters RE and RW. Supposing a voice currentcontaining all of the usual frequencles of the voice range to approachstation S from line W, at the station S it enters the output ofamplifier RW and the input of filter FE. The energy absorbed in theoutput of the amplifier RW is of course lost.

Approximately one half of the frequencies are suppressed in filter FE,which may be of any suitable type, but the other half reaches the inputof amplifier RE and are amplified thereby. The amplified current goesonto the line E after passing through the impedance looking into theinput end of filter FW, which may be of any suitable type and should bedesigned, as for instance in a way indicated hereinafter, tively low forthese frequencies. The filter FW is designed to suppress thesefrequencies to prevent them from reaching the repeater RW. The currentreaching linerE then only contains a part of the frequencies which leftline lW, but the received frequencies are sufficient for satisfactorytransmission of speech.

The operation of transmitting from line E to line W is of course thesame except that-.

to be comparathe transmission is through lter FW and amplifier RW andthe filter FW passes only `those frequencies which are not passed byfilter FE.

Various distributions of frequency bands may be employed. For instance,the filter FE may be designed to suppress all voice frequencies above1500 cycles per second, l and the filter FW to suppress all voicefreuencies below 1500 cycles or vice versa.

r one of the filters may suppress from 500 to 900 cycles and all voicefrequencies above 2000 cycles while the other filter is designed tosuppress from 0 to 500 cycles and from 900 to 2000 cycles. Or one of thefilters may suppress from 300 toOO cycles, and from 1100 to 1600 cyclesan all voice frequencies above 2700 cycles while the other filtersuppresses from 0 to 300'cycles, and from 600 to 1100 cycles and from1600 to 2700 cycles.

Tests indicate that the articulation is materially increased when themore elaborate complementary filters are used instead of merely a highpass filter and a low pass filter. The number of frequency bands to usedin each group, or in each channel, is limited at the lower side by therequired degree of naturalness and articulation and at the u pper sideby the fact that more bands require more complicated filters and causeincreased losses, due to the overlapping of the attenuation bands of thefilters in one channel with the attenuation bands of the filters in theother channel.

The repeater circuit of this invention requires no impedance balance,and permits of greater transmission gains than .do circuits in which thegain is limited by considerations of impedance unbalance. The system isnot aected by any irregularities in line impedance. An important 'fieldof application of the invention is in its use in a cord circuit vrepeater between trunks having irregular im- \pedance characteristics orin an emergency cord .circuit repeater in an exchange forconnecting'trunks in which temporary impedv 3: ance irregularities havebeen introduced accidentally. Another important field of appli cation ofthe invention is in its use in connecting trunks which are not intendedfor repeater operation, over which it is at times desirable to giverepeater service. Still another importantfield of application of theinvention is in its use in repeating 'in long temporal` lines, as forinstance` in Army wor The permissible gain in volume of ,transmissionobtainable in any case depends y wir the minimum attenuation of thefilter or FE for the fr uenciespassed by the filter FE or the filter W,respectively.

If desired the impedances looking into the West ends of the two separatechannels at the repeater station S in Fig. 1 can be connected to line Win parallel with each other rather than in series. Similarl the imedances of the two channels, Jlookin into t eir East ends, may beconnected in parallel. Further, if desireda art of the lter .in eachchannel can be placed ateach side of the mpler in that c annel, thefilters hav- *ing been shown wholly on the input side of the amplifiersfor the reason that there is then lesstendency to overloading oftheamplifiers.

Fig. 2 shows a circuit arrangement whereby both channels at the re eaterstation are served by the same amp Filters 1 and 2 are alike, and eachmay be of any of the types of filters which are intended to berepresented by FE in Fig. 1. Filters 3 and 4 -are alike, and each may beof any ofthe types of filters which are intended to ibe re resented byFW in Fig. 19 Theflters 1 anh 2 may transmit any part of the voicefrequency ranffe which any of the filters represented by FE in Fig. 1may transmit, and the filters 3 and 4 may then'.

transmit the remaining part of the voice frequency range.

In the operation of the system of Fig. 2, when 4a voice currentcontaining all of the usual frequencies of the Voice range is bein Ltransmitted East in line W, this curren asses from line W through theimpedance ooking into the output of filter 4, and to the input offilter 1. Filter 1 asses only approximately one-half of theserequencies. Filter 4 passes the other half, but the ener of this otherhalf is wasted in the imp ance looking into the output of repeater R andthe impedance looking into the input end of filter 2, since the repeateris unilateral current transmitting element and the filter 2 i's designedto suppress this other half of the voice frequencies. The frequenciespassed by filter 1 pass on through the impedance looking into the outputend of filter 3, to the input of amplifier R, are amplified by theamplifier and are then'passed through the impedance looking into theinput end of filter 4, and through filter 2 and the impedancelookingdnto the input end of filter 3, to line E. ilter 3 will not passthese .frequencies. The impedances looking into the output end of filter4, the output end of lter 3, the input end of filter 4 and the input endof filter 3 should be designed, as for instance in a way indicatedhereinafter, to be low for these frequencies. The operation oftransmitting from line E to line W is of course the same, except thatthe transmission is through filters 3 and 4 instead of through filters 1and 2, and the filters 3 and 4 pass only those frequencies which are notpassed by filters 1 and 2.

In Fig. 3 two oppositely for transmitting complementary Vfrequencyranges are both served by the saine amplifier R, as is the case in thesystem of Fig. 2. Each of the filters F, and F1 of Fig. 3 transmits thesame part of the voice frequency ran e that! fi ters 1 and 2 of Fi 2transmit. transmits the saine part of the voice freach of the filters F2and z" iio directed channels'l quency range that filters y3 and 4 ofFig. 2

transmit.

In the operation of the system of Fig, 3,

when a voice current containing all of the er, designated R. and theoutput of filter F, v in parallel.

l t e repeater and passed of these Lacanau y Filter F, asses onlyaplproxiniately one half proxima-tely one hal equencies. ilter F2 passesthe other half, but the energy of this other half is wasted in theoutput of repeater R and ,the impedance looking into the input end offilter F,, since the repeater is a unilateral current transmittingelement and the filter F is designed to suppress this other half of thevolce fre uencies. The freuencies passed by filter through filter F,' toline E. The filter F2, like the filter F2', will not pass thesefrequencies. The impedances looking into the input end of filter F2',the output end of filter F, the outtput end of filter F2 and the inputend of lter- F2 should be designed, as for instance in a wayindicatedhereinafter, to be high for these frequencies to avoid 4large losses atthese fre uencies. The operation of transmitting rom line E to line W isof course the same as that described for transmission East, except thatthe transmission is through filters F2 and F2 instead of throu h filtersF, and F,', and the filters F2 and 2 pass only those frequencies whichare not passed by filters F1 and F,'. In Fig. \4 a line L serves forduplex transmission of vo'ice currents between lines L,W and L,E and atthe same time for duplex transmission of voice currents between linesLZE and L2W. In order to accomplish this result, filters AF AF,' AF andAF2 are inserted in lines L,W, L,E, ll2E and L2W, respectively, thesefilters being similar to the correspondingly designated filters F F1',F, and F2', respectively, of Fig. 3, but having their terminal reactanceelements at those ends of the filters which are the more remote fromline L consisting of a series arm the impedance of which is, as usual,half the impedance of a full series arm.

In the operation of this system, when a voice current containing all ofthe usual voice frequencies ori inates in line L,W apof thesefrequencies is suppressed b filter AF,. The other half is transmittethrouvh filter AF line L and filter AF, to line ,E. Filters AF2 and AFaexclude the latter frequencies from lines L2W and L2E respectively. Theimpedances looking into the output end of filter AF2' should be highforthese frequencies to avoid large losses at these frequencies. Thefrequencies which filters AF, and AF, are designedto passare of coursealso excluded from lines L2E and L2W by the filters AFz and AF2',respectively, when' these frequencies are being transmitted from lineL,E through filter`AF,, line L and filter AF, to line L,W. i l

When a voice current containing all ofl Vthe usual voice frequenciesoriginates iin line L,W approximately one half of these are amplifiedby4 FE be made low and the input end of filter AF,`

frequencies is suppressed by filter AF,'. The other half is'transmitt'edthrough filter AF2', line L and filter AF2 to line L,E. Filters AF, andAF, exclude the latter fre uencies from line L,W and L,E, respec-V tivey; The impedances looking into the output ends of filters AF1 and AF,'should be high for these frequencies to avoid lar'ge losses at theseVfre uencies. The frequencies which filters F2 and AF2' are designed topass are ofcourse also excluded from lines L,E and L,W by the filtersAF,' and AF, respectively, when these frequencies are being transmittedfrom line L E through alter AF2, une L and alter AFO in series insteadof the impedances looking into their West ends being connected inparallel and the impedances looking into their East ends being connectedin parallel as are the corresponding impedances of filters AF, and AF2Iin Fig. 3. The'filters AF2 and AF,' in Fig. 4 may have the irnpedanceslooking into their West ends connected either in series or in parallel,regardless of Whether the filters AF2 and AF,' have the impedanceslooking into their West ends connected in parallel for in series.

The impedance looking into the input of filter FW of Fig. 1 may bemadelow for the frequencies passed by filter FE and the impedance'looking into the input of filter for the frequencies passed by filterFW, by 4properly terminating these filters at their input ends. Forinstance if FE be a low pass filter suppressing all frequencies above1500 and FW be a high pass filter suppressing all frequencies below1500, then assuming each of these filters to be made up of, forinstance, a plurality of sections each section of filter FE consisting,say, of a series inductance element and a shunt capacity element afterthe fashion of the filter of Fig. 7 of U. S. Patent to G. A.

Electrical receiving translating, or repeating circuit, and each sectionof filter .FV

consisting, say, of a series capacity element and a shunt inductanceelement after the fashion of the filter of Fig. 6 of the CampbellApatent mentioned above, then the first section of high pass filter FWmay be made 'to begin with a shunt inductance element having` an`impedance which is 10W for the frequencies passed by low pass filter FE,and the first section of low pass filter FE i mentioned above, andfilters 3 and '4 to then be high pass filters suchas that of i;

of the Campbell patent, then each en high pass filters 3 and 4 may beterminated in a shunt inductance element of low impedance for thefrequencies passed by low pass filters 1 and 2, and each end of low passfilters 1 and 2 maybe terminated in a shunt condenser of low `impedancefor the fre quencies passed by high pass filters 3 and 4. Y It will beclear that when one end of a low pass filter is connected in parallelwith 4one end of a high pass filter, as for instance in providing asystem such' as is shown in Figs. 3 and 4, then the one end of the highpass' filter may be made' to begin with a series condenser having highimpedance vfor` the frequencies passed by the low pa`ss filter,

susv

- of the and the one end of the low\pass filter may be made to begin.with a series inductance element having high impedance forl thefrequencies passed by the hi h pass filter.

Preferably where it is esired to connect a high ass filter such as isshown in Fig. 6 Campbell atent referred to above in parallel with a owpass filter such as is shown in Fig. 7 of that patent, for separatingcurrents into two channels, the filters should have4 approximately thesame cut-off fre uency and, using the symbols employed in t e patent,Z1, Z2 should be constant with L2 varymg frequency and C: should beequal to L1 2 nated with a series condenser having a value ofapproximately 1.25 C at the end which is to' be connected in parallelwith the second filter and the second filter at the corresponding endshould be terminated in a series reactance coil having an inductance of.8 L1. If the ends of the filters are to be connected in series thefirst filter should end in a shunt element having an inductance of 1.25L, and the second should end in a shunt element having a capacity of .8C2. If this is done the impedances of the filters will be such that thecombination will have a practically constant impedance at allfrequencies except in the immediate vicinity of the cutolf fre uency andneither filter will interfere deleteriously with the operation of theother. The principles and reasons underl in this design of the filtersare set forth 1n etail in the United States Patent to Otto J. Zobel,'No.1,557,230, dated October 13, 1925, entitled Complementary filters,issued on pplication Serial No.377 ,965, filed April Then the firstfilter shouldbe termi.

' s mesma In a similar waywhen a multi-band filter l lto above theimpedance of the series arm multiplied by the im edance of the shunt armshould preferab y be constant at allr frequencies and the filter shouldpreferably be terminatedin a series section having approximately timesthe impedance of the full series section if it is to be connected inparallel with a complementary filter or in a vshunt section havingapproxi- 4mately .1.25 times the impedance ofthe full shunt section ifit is to be connected Kin series with a complementary filter. A

method ofdesignin a wave filter in which the impedance of t e seriesarmmultiplied by the impedance of the shunt arm is constant with?varying frequency, and `which will transmit over any preassigned numberofl frequency ranges is disclosed in United States Patent to Zobel, No.1,509,184, dated September 23, 1924, entitled Multiple band wave filter,issued on application Serial No. 377,963, filed April 30, 1920..

AAs noted above, a part of the filter in each channel'in Fig-1 can beplaced at each side of the amplifier in .that channel after the generalfashion in which the filtering means 1 and 2 inFigs2 is placed partlyat-each side of the amplifierl Then the resultant ofthe impedanceslooking from line W into the two channels, and theresultant of theimpedances looking from the lou line E into the two channels may bemadel practically constant at all fr uencies except in theimmediatevicinity o thacut-o frequencies, in the manner explained above.

What is claimed is:

1. Thev method oftreatin telephonie messages simu taneously impressedupon the line, each as a group of alternating currents in the voicefrequency range, .which comprises separating at the receiving end of theline groups of currents each of which groups represents one of saidmessages, and one of which oups consists of currents of frequencieslying outside the limits of the frequencies of the currents in anotherof said groups.

2. The method of operating a repeater having oppositely directedunilaterally repeating paths which comprises transmitting in one of saidpaths voice currents of frequencies limited to part of the voicefrequency range, and transmitting in the other of said paths voicecurrents of frequencies limited to the remainder of the voice frequencyrange.

' 3. -A system for repeating voice frequency currents, comprising alineand a two-way repeater circuit including transmission aths, selective ofvoice currents of certain requencies, for frequency separation of opanumber of'- pitely directed voice frequency currents transmissions fromeach other.

4.' In a voice frequency telephone system, the combination with two llnecircuits for transmitting voicel currents,I of a two channel connect-ingcircuit for transmitt' voice frequencies inF opposite directions, saitwo channels havinga common transmitting portion and `beingsimultaneously in con; dition to transmit between said two lines.

5. In a voice frequency telephone system, the `combination with two linecircuits for transmitting voice currents, of a two channel connectingcircuit for transmitting voice frequencies in opposite directions, saidtwo channels having a common unilaterally transmitting ortion normallyconnected for transmission 1n each direction between said line circuits.

6. A line for transmitting a pllurality of voice current waves ofsubstantia y different voice frequencies in each direction, a secondsimilar` line, and two-way repeating means capable of repeating saidwaves simultaneously without mutual interference between said lines,said means com rising a single am lifyingfpath associate with both ofsai lines. i

7. Two lines fortransmitting voice frequency currents in bothdirections, and an am lifier having an input circuit coupled to bot ofsaid lines, and an output circuit cou- \pled to one of said linesthrough a path selective of certain voice frequencies and to 35 theother line through aa pathselective of only other voice frequencies.

v8. Two lines for the transmission of voice frequency currents in bothdirections, a oneway selective path between said lines for transmittingvoice frequency currents having certain characteristlcs, another one-wayselective path between said lines for transmitting in the oppositedirection only voice frequency currents of characteristics differentfrom the first mentioned characteristic and a single amplifier common toboth sai paths'. p

9. Two lines for the transmission of voice frequency currents in eachdirection thereover, the voice currents transmitted through bothof saidlines in one direction -having different characteristics from thosetransmitted through both of said lines in the opposite direction, anamplifier for amplifying all of said currents havingI its input and itsoutput circuits each common to both of said lines, anda yplurality ofselective. paths in said output circuit between said amplifier and eachof said lines for selectively transmitting said currents.

10. Two lines for the transmission of voice current waves in eachdirect-ion thereover the voice current waves transmitted over both ofsaid lines in one direction having respectively' differentcharacteristics from those opposite direction, an am lifier for all ofsaid waves, an input circuit and an output circuit each having twobranches one for each line, and lilteis in each of said branches.

11. A line, a two-way repeater in said line comprising a circuit fortransmission of voice frequency currents in one direction, and anothercircuit for transmission of currents of other different voicevfrequencies in the opposite direction, an amplifier in one of saidcircuits, another amplifier in said other circuit, and selective meansin one of said circuits for selecting the frequencies used intransmitting in one direction, and selective means in the other circuitfor selecting the frequencies used in transmitting in the oppositedirection.

l2. In a telephone system, transmission line sections, one-watransmission paths interconnecting said ine sections, and`selectivemeans whereby only a portion of the voice frequencies approaching saidpaths from one direction over said line sections will pass over one ofsaid paths, and whereby o the voice frequencies approaching said pathsfrom the op osite direction only frequencies different rom those passedby said one path will pass over the other of said paths. v

13. In a telephone system, transmission line sections, one-way.transmission paths interconnecting said line sections,.oneway repeatersin each of said paths and selective means in one of said paths wherebyonly a portion of the voice frequencies approachmg said paths over saidline sections 1n one direction will pass over said one path, and se-Vlective means in the other path whereby of the voicefrequencies'approaching said aths over said line sections in theopposite direction only fre uencies different from those passed by saione path will pass over said other path. A

14. A channel comprising a band filter for transmitting voicefrequencies lying between certain limits, a second channel comprising asecond band filter for sup ressing frequencies between said limits, eacfilter being capable of passing voice currents representing intelligiblespeech, and a line having one end connected to one end of the firstmentioned channel and to one end of said second channel, the impedanceof said second channel looking from said end of said lineand said end ofsaid first channel being of such value for the frequencies passed bysaid first chan' nel that the volume loss in transmission due to said'im edance is small for frequencies transmitte through said line andsaid first filter. l

15. A common transmission path, a plurality of'channels each adapted totransmit currents of different frequency from those transmitted by theother channels, and s transmitted over bothof said lines in the icoseparate .band filter transmitting voice quencies" connectin eachchannel to vsaid path the terminations of said filters elecjacent saidpath being connected in parallel.

16. A common transmission path, a plurality of channels each adapted totransmit currents of different frequency from transmitted by the otherchannels, and a separate band lter capable of' passing voice currents reresent'ing intelligible speech connecting eac channel to said path,.theterminations of said filters electrically adjacent said path beingconnected in parallel.

17. A circuit a plurality of voice current channels assoclatedtherewith, means :for

, transferring voice currents of different voice frequencies betweensaid channels and saidv circuit, said means comprising an individuallter connected to each of said channels, the transmission range of eachfltervbeing distinct and being such as to permit p e of voice currentsrepresentin intelligib e speech, said filters having termmationsconnected in parallel to said circuit, the termina tion of each filterhaving a high impedance Vfor currents of the transmission frequencyVrange of any other filter whereby said filters do not short-circuiteach other.

30 18. A common transmission path, two

and a second filter capable of 1passinghvoice n e speec concurrentsrepresenting mtelligib necting the second of said channels'to said adifferent .range of' voice path, each of said filters comprisingsections aving shuntand series arms, ea'ch filter being terminated in aseries arm having approximately .8 times the impedance of a full seriesarm, 'and the terminations of said filters being connected to said pathin parallel. 19. The method of two-way telephone transmission of speechcurrents over the same line com rising transmitting in one directionover sai line currents in the ordinary/speech fre uency 'range confinedto frequencies hig er thanA a given limiting frequency, and transmittingin the opposite direction over said `line currents in the ordinaryspeech frequency range confined to frequencies lower than said givenlimiting frcquenc In witness whereof, IA hereunto su scribe my name this24th day of September A. D.,

FREDERICK WHITI'LESEY McKOWN.

